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This paper presents a UHF-RFID reader antenna radiating circular polarization with unidirectional pattern. The antenna structure consists of two plates. The upper plate acted as radiating element contains notched rectangular plate, rectangular aperture and slot line. The lower plate is the ground plane. The parametric study is carried out to achieve the frequency response of 920-925 MHz according to Thailand standard of UHF-RFID system. The prototype antenna was fabricated and measured. The simulated half-power beamwidth (HPBW) in E- and H-planes are 65 and 68 degree, respectively. The measured HPBW in both planes is identical to 55 degree. The simulated and measured bandwidths are 7.26% and 10.7%, respectively. The simulated and measured axial ratios at 922.5 MHz are 0.64 dB and 0.09 dB, respectively.